Surface modified pigments

ABSTRACT

.Iadd.Finely divided particulate inorganic pigment is surface modified with from about 1% to about 15% of an amino organosilane, particularly gamma-aminopropyltriethoxy silane. Thermosetting resins incorporating such modified inorganic pigments exhibit improved physical properties.

This is application is a .Iadd.reissue of U.S. Pat. No. 3,290,165, Ser. No. 269,695 filed April 1, 1963, which is a .Iaddend.continuation-in-part of applicant's copending application Ser. No. 189,321, filed April 23, 1962, entitled "Surface Modified Pigments," and now abandoned.

This invention relates to finely divided particulate inorganic pigments modified with amino organosilanes and the process for their production.

When inorganic pigments are modified with the silanes according to this invention, the properties imparted to them are such that they can advantageously be used as fillers for thermosetting resins such as polyurethanes, epoxy polymers, melamine polymers, phenolic polymers, ureaformaldehyde polymers, unsaturated polyesters, as well as other polymers and elastomers including polyethylenes, polypropylenes, polystyrenes, saturated polyesters, polyamides, polyvinyl compounds, polyisoprenes, polybutadienes, polystyrenebutadienes, and the like.

The modified pigments can also be advantageously used as fillers for paper, paints, varnishes, inks, and paper coating compositions.

By the use of these modified finely divided particulate inorganic pigments, improved physical properties are imparted to the vehicles into which they are incorporated.

Inorganic pigments modified with amino organosilanes have affinity for direct dyes and are useful for imparting various colors to the vehicles.

An object of this invention is to provide modified pigments especially useful as fillers.

Another object of this invention is to provide modified pigments which are dyeable with direct dyes and are useful as color-imparting fillers.

A further object of this invention is to provide modified pigments which can be used as fillers in applications where they had heretofore been unsatisfactory.

A still further object of the invention is to provide cross-linkable fillers capable of imparting improved abrasion resistance among other improved properties to elastomers.

Other objects and advantages will be apparent from the following specification.

I am aware of extensive efforts in the prior art to improve properties of filler pigments by modification with organosilanes. Hydrocarbon silane modifications of pigments do impart improved dispersions in organic vehicles but such modifications do not normally increase reinforcement in vinyl addition polymers unless the hydrocarbon silane carries specific types of unsaturation which serves to promote a more tenacious bridge between the filler and the vehicle. In any case, all these prior art modified pigments are rendered hydrophobic by modification with either saturated or unsaturated hydrocarbon silanes and, furthermore, such silane modified pigments are not valuable reinforcing fillers in saturated thermosetting resins.

I have discovered that modification of filler pigments with saturated amino organosilanes improves reinforcement in a wide variety of vinyl addition as well as thermosetting polymers in that a strong chemical bridge between filler pigment and polymer results. Bridging is accomplished through the amino modified surface of the pigments. Surprisingly, amino organosilane modification improves reinforcement in both vinyl addition and thermosetting polymers, whereas unsaturated organosilane fillers are generally only effective in vinyl addition polymers where unsaturation is present. Another important advantage of amino organosilane modified fillers is that they are usually hydrophilic whereas prior art silane modified fillers are hydrophobic.

The modified pigments of this invention can be prepared by dissolving the desired amount of amino organosilane in a suitable solvent, adding the pigment and heating until the reaction is complete. The amount of modifier added depends upon the particular pigment being modified and the use for which it is intended. Generally up to about 15% by weight of the modifier is sufficient for most purposes.

A particularly useful process of modifying pigments according to this invention involves spray drying pigment slurries having one or more of the amino organosilanes dispersed therein. The spray drying process effects a uniform distribution of the modifier on the pigment and virtually instantaneously cures the modifier on the pigment.

The compounds used to modify the pigments can be depicted by the formula: ##STR1## wherein R₁ is hydrogen, alkyl, aryl, cycloalkyl, or alkylaryl; R₂ is hydrogen, alkyl, aryl, cycloalkyl or alkylaryl; R₃ is hydrogen, lower alkyl, aryl, lower alkylaryl, or loer arylalkyl; R₄ is hydrogen, lower alkyl, aryl, lower alkylaryl or lower arylalkyl; R₅ is hydrogen, lower alkyl, aryl, lower alkylaryl or lower arylalkyl; and X is alkylene, arylene, alkylarylene, arylalkylene, cycloalkylene having secondary and/or tertiary nitrogen present in the chain, and/or primary, secondary, and/or tertiary nitrogen pendant from the chain. Some of these amino organosilanes are disclosed along with methods for their preparation in U.S. Pat. Nos. 2,832,754; 2,930,809; 3,007,957; and 3,020,302. Commercially available amino organosilanes include "A-1100" (gamma aminopropyltriethoxysilane) and "Y-2967" (an amino silane which is a modified gamma aminopropyltriethoxysilane) sold by Union Carbide Corporation, N.Y., N.Y., and "Z-6020" (a diamino functional silane) sold by Dow Corning Corporation, Midland, Michigan.

.Iadd.Preferred compounds to modify the pigments are amino organosilanes of the formula:

    H.sub.2 N--R--Si(OR').sub.3

wherein R is selected from the group consisting of phenylene, lower alkyl substituted phenylene, lower alkoxy substituted phenylene, and lower alkylene, R' is monovalent hydrocarbon group free of aliphatic unsaturation selected from the group consisting of lower alkyl, aryl, lower alkaryl and lower aralkyl, wherein R' can represent the same or different groups. .Iaddend.

Pigments advantageously modified in the practice of this invention are finely divided particulate inorganic pigments such as, for example, inorganic compounds of silicon, including hydrated or anhydrous silicas, calcium silicates, magnesium silicates, calcium-magnesium silicates, barium silicates, aluminum silicates, sodium-alumino-silicates, calcium-alumino-silicates, calcium-sodium alumino silicates; clays such as kaolins which include dickite, kaolinite and nacrite, halloysite, montmorillonites including sodium and magnesium bentonites, synthetic or natural zeolites; various metal oxides and carbonates such as zinc oxide, alumina, titania or magnesia, calcium carbonate; and various non-white pigments like carbon blacks, zinc sulfide, ferric oxide and the like.

All the above fillers are available on a commercial scale and include the following, all of which are finely divided, particulate substances.

Zeolex®, very finely divided precipitated sodium alumino silicate pigments of submicron particle size and disclosed in U.S. Pat. Nos. 2,739,073 and 2,848,346.

Zerosil®, very finely divided precipitated hydrated silicas of submicron particle size and disclosed in copending U.S. Pat. applications Ser. No. 144,168 filed Oct. 10, 1961, and 149,964 filed Nov. 3, 1961.

Suprex®, an air floated kaolin clay with platelike particles of which 87-92% are minus 2 microns.

Aromex®, intermediate super abrasion furnace carbon blacks.

Essex®, semi-reinforcing furnace blacks.

Silene EF®, a precipitated hydrated calcium silicate of very fine particle size.

Hi-Sil®, a precipitated hydrated silica of very fine particle size.

Celite®, a diatomaceous earth which is principally a hydrated silica.

Alumina C®, a hydrated aluminum oxide of small particle size.

Kadox®, a zinc oxide filler.

Titanox®, a pigment grade commercial titanium dioxide.

Cab-O-Sil®, a very finely divided anhydrous silica.

Ludox®, a precipitated silica of very fine particle size.

The following examples illustrate typical methods by which various pigments are surface modified in accordance with this invention.

Example 1

8 grams of gamma aminopropyltriethoxysilane (A-1100) was dissolved in 3.3 liters of benzene in a 5-liter round bottom flask. 400 grams of carbon black (ISAF) was added and the resulting mixture was refluxed 2 hours. The resulting product contained 2% of the modifier based on the weight of the carbon black.

Example 2

"Suprex" was modified with 1.0% by weight with gamma aminopropyltriethoxysilane by adding the appropriate amount of the modifier using water as a solvent and then adding the clay and refluxing for 21/2 hours. The products were recovered and dried. The example was repeated with 2.0% and 3.0% gamma aminopropyltriethoxysilane.

Example 3

"Zeolex 23" was modified with 1% by weight with gamma aminopropyltriethoxysilane by adding the "Zeolex" to a benzene solution of the modifier and refluxing for 21/2 hours. The product was recovered and dried.

Example 4

"Suprex" was modified with 1.0% of "Z-6020" by adding 3.33 pounds "Z-6020" to 667 pounds of water while under agitation. 333 pounds of "Suprex" was slowly added to the solution while continuing the stirring until a homogeneous clay slip resulted. The clay slip was then spray dried in a 7-foot conical spray dryer operated at an inlet temperature of 600° F. and an outlet temperature of 250° F. A finely pulverized, chemically modified clay product was obtained. The example was repeated to produce 2.0% and 3.0% modifications of the "Suprex."

Example 5

Example 4 was repeated using "Y-2967" instead of "Z-6020."

Example 6

Example 4 was repeated using "A-1100" instead of "Z-6020."

Example 7

"Suprex" was modified with 1% of gamma aminopropyltriethoxysilane by adding 10 grams gamma aminopropyltriethoxysilane to 3.5 liters benzene, then adding 1 kilogram "Suprex" clay and refluxing for 3 hours. The modified clay was recovered and dried. This example was repeated using 2% and 3% instead of 1% gamma aminopropyltriethoxysilane.

Example 8

Example 7 was repeated using "Z-6020" in place of gamma aminopropyltriethoxysilane.

Example 9

Example 7 was repeated using "Y-2967" in place of gamma aminopropyltriethoxysilane.

The above examples illustrate the facility with which various inorganic pigments are modified with amino organosilanes.

The examples were repeated using each of the pigments named herein to produce modified pigments having properties similar to those discussed below.

While only three modifiers are exemplified, this is done for convenience since all those disclosed herein have been used for the purpose and come within the scope of this invention.

The physical properties of the various pigments disclosed herein are significantly altered by modification with the group of silanes disclosed herein. For example, when kaolin clay is so modified, a dramatic change in its properties is apparent. Where, before, the clay lacked significant affinity for direct dyes, it is modified by the process of this invention to be readily dyeable with direct dyes. The modified kaolin clays can be used as a filler for polyurethanes where, before modification, it was unusable since it prevented a cure of the polymer. This is illustrated in Table I in which the following formation was employed:

    ______________________________________                                                        Parts                                                           ______________________________________                                         Vibrathane 5003.sup.1                                                                           100                                                           Stearic acid     0.25                                                          Di-Cup 40C.sup.2 5                                                             Clay             60                                                            ______________________________________                                          .sup.1 A polyurethane produced by Naugatuck Chemical Division of U.S.          Rubber Company.                                                                .sup.2 A polymerizing crosslinking agent produced by Hercules Powder           Company.                                                                 

The compounds were mixed on a 6-inch by 12-inch laboratory mill and cured for 30 minutes at 307° F., except for the NBS abrasion test where the cure was for 60 minutes at 307° F.

                                      TABLE I                                      __________________________________________________________________________                                Example 2                                                                      Suprex                                                                              Suprex                                                                              Suprex                                                               plus 1%                                                                             plus 2%                                                                             plus 3%                                                    Control                                                                             Suprex                                                                              Modifier                                                                            Modifier                                                                            Modifier                                  __________________________________________________________________________     Parts filler/100 parts polymer                                                                  None 60    60   60   60                                       Tensile, p.s.i   5,240                                                                               No cure                                                                             3,680                                                                               3,770                                                                               3,840                                     Stress, 300%, p.s.i                                                                             830  No cure                                                                             2,070                                                                               3,190                                                                               --                                        Elongation, percent                                                                             500  No cure                                                                             470  425  265                                       Shore A Hardness g                                                                              56   No cure                                                                              71   71   75                                       NBS Abrasion, percent of standard                                                               129  No cure                                                                             122  172  202                                       __________________________________________________________________________

The results illustrate the improved properties of modified kaolin clay filled polyurethane over both the compound filled with unmodified kaolin and the unfilled compound. Note, for example, the increase in abrasion resistance with increased modification of kaolin. It is also apparent from the data that unmodified kaolin is unsatisfactory as a filler for polyurethanes since the polymer did not cure. The use of modified kaolin clay not only improves the properties of the polyurethane but also decreases the raw material cost since the filler is much less expensive than the polymer.

                                      TABLE II                                     __________________________________________________________________________     MODIFIED SUPREX CLAYS IN VIBRATHANE 5003                                                                          Example 2                                                       Minutes   Suprex                                                                              1%   2%   Example 4                                                                            Example                                                                              Example 9                                 Cured at                                                                            Unfilled                                                                            Filled                                                                              Modifier                                                                            Modifier                                                                            1% Z-6020                                                                            1% Z-6020                                                                            1% Y-2967             Physical Properties 305° F.                                                                      Control                                                                             Control                                                                             Water                                                                               Water                                                                               Water Benzene                                                                              Benzene               __________________________________________________________________________     200% Modulus        30   430  1,270                                                                               1,340                                                                               2,900                                                                               --    2,900 1,710                                     60   500  1,310                                                                               1,530                                                                               2,990                                                                               2,890 1,710 --                                        75   510  1,330                                                                               1,480                                                                               2,820                                                                               --    2,860 1,840                 300% Modulus        30   830  1,570                                                                               2,070                                                                               3,190                                                                               --    3,270 2,060                                     60   1,050                                                                               1,640                                                                               2,220                                                                               3,340                                                                               3,330 3,270 2,090                                     75   1,040                                                                               1,630                                                                               2,520                                                                               3,200                                                                               --    3,240 2,170                 Tensile Strength    30   5,240                                                                               4,340                                                                               3,680                                                                               3,770                                                                               2,960 3,470 4,040                                     60   4,890                                                                               3,850                                                                               3,920                                                                               3,490                                                                               3,620 3,290 3,610                                     75   5,320                                                                               3,640                                                                               3,560                                                                               3,510                                                                               --    3,240 3,950                 Elongation          30   500  570  470  425  190   365   560                                       60   440  525  450  335  360   315   500                                       75   450  505  440  370  --    300   510                   Hardness, Shore A   30   56   72   71   71   75    75    70                                        60   59   74   73   73   76    76    71                                        75   59   74   73   73   13    76    71                    Crescent Tear       30   65   285  280  238  225   235   303                                       60   68   270  230  243  205   193   220                                       75   73   243  231  225  --    193   225                   NBS Abrasion Index, Percent                                                                        60   74.8 63.6 77.2 109.5                                                                               143.5 131.4 68.2                                      75   80.3 62.2 87.4 137.0                                                                               105.5 152.1 81.5                  Hardness, Shore A, NBS Specimens                                                                   60   55   71   72   75   76    76    71                                        75   56   73   73   75   76    76    72                    NBS Abrasion (Gum=100%)                                                                            60   100  85   103  147  192   176   91                                        75   107  83   117  183  141   204   109                   Compression Set "B" 22 hrs./158° F.                                                         60   5.5  34.0 17.5 11.3 10.1  12.0  25.0                                      75   5.1  36.7 16.5 12.0 9.5   11.0  23.9                  Mooney Viscosity, ML 4'/212° F.                                                             --   44   60   65   65   83    65    64                    Mooney Scorch, MS/265° F.                                                                   --   23   26   20   16   12.5  18    23                    __________________________________________________________________________

Table II demonstrates dramatic improvements in properties of polyurethane filled with amino organosilane modified clays.

When modified carbon black is used as the filler in a rubber recipe, good results compared to unmodified black are achieved with a 2% by weight modification using gamma aminopropyltriethoxysilane. The results listed in Table III are based upon tests in the following recipe.

    ______________________________________                                                       Parts/100 RHC                                                    ______________________________________                                         Smoked sheet    100.0                                                          ISAF carbon black                                                                              45.0                                                           Zinc oxide      3.0                                                            Stearic acid    3.0                                                            Pine tar        3.0                                                            Age rite HP.sup.1                                                                              1.0                                                            NOBS special.sup.2                                                                             0.35                                                           Sulfur          2.75                                                           Total           158.10                                                         ______________________________________                                          .sup.1 An antioxidant containing phenylbeta-naphthylamine and                  N,Ndiphenyl-para-phenylenediamine.                                             .sup.2 Accelerator containing Noxydiethylene benzothlazol2-sulfenamide.  

The batches were mixed on a Banbury using speed #1, ram pressure of 30 p.s.i., and a starting temperature of 125° F.; the final batch mix was on a 6-inch by 12-inch mill and the inlet water temperature was 158° F. The compound was cured for 70 minutes at 275° F., then tested. The results are listed in Table III.

                  TABLE III                                                        ______________________________________                                                                               Abrasion,                                             Percent  Modulus, Tensile,                                                                              Huber-                                   Pigment      Modifier p.s.i.   p.s.i. Williams                                 ______________________________________                                         ISAF Carbon Black                                                                           None     1,780    4,590  100.0                                     Control.                                                                      ISAF Carbon Black                                                                           .sup.1   1,970    4,720  107.9                                    ______________________________________                                          .sup.1 2.0% gamma aminopropyltriethoxysilane.                            

This data indicates that when carbon black is modified with controlled amounts of modifier, the properties which it imparts to rubber are improved in respect to modulus, tensile, and abrasion resistance.

When modified Zeolex is used as a filler for rubber compounds, it imparts to the rubber improved properties of modulus, tensile strength, tear resistance and abrasion resistance when compared to these same properties in rubber filled with unmodified Zeolex. The results in Table IV are based upon the following recipe:

    ______________________________________                                                      Parts/100 RHC                                                     ______________________________________                                         GRS 1502.sup.1 100.0                                                           Pliolite S6B.sup.2                                                                            20.0                                                            Zinc oxide     3.0                                                             Stearic acid   2.0                                                             Cumar MH 21/2.sup.3                                                                           7.5                                                             Zeolex 23      66.5                                                            Santocure.sup.4                                                                               2.0                                                             DOTG.sup.5     1.0                                                             Sulfur         2.5                                                             Total          204.5                                                           ______________________________________                                          .sup.1 Emulsion copolymer of 23.5% styrene and 76.5% butadiene.                .sup.2 A styrenebutadiene copolymer of high styrene content.                   .sup.3 Paracumarene-indene resin.                                              .sup.4 nCyclohexyl-2-benzothiazole sulfenamide accelerator.                    .sup.5 Diortho-tolylguanidine.                                           

The recipe was mixed on a Banbury mixer at speed #1, ram pressure of 30 p.s.i., and at a starting temperature of 125° F. The final batch was mixed on a 6-inch by 12-inch mill with a water inlet temperature of 158° F. The compound was cured at 292° F., then tested. The results are shown in Table IV.

                  TABLE IV                                                         ______________________________________                                         Cure    200%     300%     400%                                                 Minutes Modulus  Modulus  Modulus                                                                               Tensile                                                                              Elongation                              ______________________________________                                         PIGMENT-ZEOLEX 23 UNMODIFIED-CONTROL                                           5        80      --       --     80    280                                     10       80      --       --     80    280                                     15      470      650      860    1,300 605                                     20      720      1,000    1,350  1,640 460                                     30      750      1,060    1,450  1,560 420                                     ______________________________________                                         PIGMENT-ZEOLEX 23 MODIFIED WITH 10% GAMMA                                      AMINO-PROPYLTRIETHOXYSILANE                                                    5       690      1,010    1,340  1,970 580                                     10      910      1,300    1,710  2,480 550                                     15      1,010    1,400    1,820  2,360 510                                     20      1,070    1,480    1,920  2,280 470                                     30      1,090    1,480    1,940  2,360 480                                     ______________________________________                                                      Abrasion Index.sup.1                                                                      Shore Hardness                                         Pigment        10'    15'    20'  10'  15'  20'                                ______________________________________                                         Zeolex 23 Control                                                                             .sup.2 41.5   47.5 60   72   77                                 Modified Zeolex 23                                                                            61.7   63.0   62.3 76   76   77                                 ______________________________________                                                      Tear Resistance, Avg.                                             Pigment        5'       10'      15'   20'                                     ______________________________________                                         Zeolex 23 Control                                                                             37.5     38.5     174   160                                     Modified Zeolex 23                                                                            216.5    193.5    195   187.5                                   ______________________________________                                          .sup.1 Percent of NBS Standard sample.                                         .sup.2 Not cured.                                                        

The results indicate that Zeolex 23 modified with gamma aminopropyltriethoxysilane, when compared with unmodified Zeolex 23 used as a filler for rubber, is faster curing, has increased modulus, increased tensile strength, and improved tear resistance and abrasion resistance.

It should also be noted that physical and "wet" electrical properties of filled resin systems can be significantly improved by treating the fillers in accordance with this invention.

I have found that in addition to the concepts disclosed above, the properties of the modified pigments are affected by the solvent used in their preparation.

The properties of carbon blacks, clays and silicates modified in aqueous systems, such as disclosed in Example 2, vary markedly from the properties of these same pigments modified in nonaqueous systems as disclosed in Examples 1 and 3.

In order to demonstrate these differences, regular Suprex clay, Suprex clay of Example 2, and Suprex clay modified in nonaqueous solvent according to the teachings of Example 7 were used in producing rubber compounds using the following recipe.

    ______________________________________                                                            Parts by weight                                             ______________________________________                                         Smoked sheet.sup.1   100                                                       Clay (as specified in Table V)                                                                      104                                                       Zinc oxide           5                                                         Sulfur               3                                                         Captax               1                                                         Stearic acid         4                                                         ______________________________________                                          .sup.1 Natural rubber.                                                   

The compounds were mixed on a 6-inch by 12-inch laboratory mill and then cured at 260° F. to produce 30-, 45-, and 60-minute cures of each.

Table V below compares the abrasion index and the 200% modulus level of each of the test materials.

                  TABLE V                                                          ______________________________________                                                      Abrasion Index,                                                                           200% Modulus                                                        Percent    p.s.i.                                                 Clay           30'    45'    60'  30'  45'  60'                                ______________________________________                                         Suprex Control 55.0   55.9   53.7 760  960  990                                Suprex 3% Modifier                                                                            72.8   68.6   65.2 2,000                                                                               2,070                                                                               2,170                              (Benzene)                                                                      Suprex 3% Modifier                                                                            84.6   85.5   70.7 1,740                                                                               1,970                                                                               1,990                              (Water)                                                                        ______________________________________                                    

The results as set forth in Table V clearly indicate that clays modified in accordance with the invention impart superior properties to rubber compounds when used as a filler therein. These results also demonstrate that the clays modified in an aqueous system give a higher abrasion resistance and a lower modulus than clays modified in a nonaqueous system.

While natural rubber was used in the recipes tested in Table V, these tests were also conducted with similar results from recipes using SBR, polyurethanes and polybutadiene.

The foregoing is illustrative only and additional modifications may be made without departing from the substance of the invention as defined in the appended claims. 

I claim:
 1. A .Iadd.filler comprising a .Iaddend.finely divided particulate inorganic pigment surface .Iadd.selected from the group consisting of synthetic silicas, silicates, metal oxides, calcium carbonates, zinc sulfides, and carbon blacks, said pigment surface having been .Iaddend.modified .Iadd.by treatment .Iaddend.with from about 1% to 15%, based on the weight of the dry pigment, of an amino organosilane of the formula ##STR2## wherein R₁ is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and alkylaryl, R₂ is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl and alkylaryl, R₃ is selected from the group consisting of hydrogen, lower alkyl, aryl, lower alkylaryl and lower arylalkyl, R₄ is selected from the group consisting of hydrogen, lower alkyl, aryl, lower alkylaryl and lower arylalkyl, R₅ is selected from the group consisting of hydrogen, lower alkyl, aryl, lower alkylaryl, and lower arylalkyl, X is selected from the group consisting of alkylene, arylene, alkylarylene, arylalkylene, cycloalkylene containing secondary amino nitrogen, and cycloalkylene containing tertiary amino nitrogen.Iadd., the modification of said pigment surface by said organosilane comprising spray drying slurries of said pigment having one or more of the amino organosilanes dispersed therein. .Iaddend. .[.
 2. A compound as in claim 1 wherein the finely divided particulate filler is selected from the group consisting of synthetic silicas, silicates, metal oxides, calcium carbonates, zinc sulfides, and carbon blacks..].
 3. A compound as in claim 1 wherein the amino organosilane is gamma aminopropyltriethoxysilane.
 4. A compound as in claim 1 wherein the amino organosilane is a diamino functional silane.
 5. Finely divided particulate hydrated silica, surface modified with from about 1% to 15% by weight based upon the weight of the silica of gamma aminopropyltriethoxysilane.
 6. Finely divided particulate sodium alumino silicate pigment, surface modified with from about 1% to 15% by weight, based upon the weight of the pigment, of gamma aminopropyltriethoxysilane.
 7. Finely divided particulate carbon black, surface modified with from about 1%to 15% by weight, based upon the weight of the carbon black, of gamma aminopropyltriethoxysilane.
 8. Finely divided particulate kaolin clay, surface modified with from about 1% to 15% by weight, based upon the weight of the kaolin clay, of gamma aminopropyltriethoxysilane. .Iadd.
 9. A filler comprising a finely divided particulate inorganic pigment surface selected from the group consisting of synthetic silicas, silicates, metal oxides, calcium carbonates, zinc sulfides, and carbon blacks, said pigment surface having been modified by treatment with from about 1% to 15%, based on the weight of the dry pigment, of an amino organosilane of the formula

    H.sub.2 N--R--Si(Or').sub.3

wherein R is selected from the group consisting of phenylene lower alkyl substituted phenylene, lower alkoxy substituted phenylene, and lower alkylene, R' is a monovalent hydrocarbon group free of aliphatic unsaturation selected from the group consisting of lower alkyl, aryl, lower alkaryl and lower aralkyl, wherein R' can represent the same or different groups. .Iaddend. 